U.S. Pat. No. 5,715,263 issued to SDL describes an example of a laser shown in FIG. 2 of this patent comprising a quantum well laser 26 with an output mirror 27 outputting into an optical fiber 32. This type of laser is used in telecommunications to pump an amplifier outputting into a transmission line. According to the invention described in the SDL patent, the fiber 32 comprises a fiber Bragg grating 34 with the function of reflecting part of the light emitted by the laser 26 back to the laser 26. This patent (column 2, lines 37–45) describes how the optical spectrum of the emitting laser diode is affected if the center of the reflection band of the fiber Bragg grating is in the laser gain band. The exact effect depends on parameters such as the value of the reflection coefficient and band width of the fiber Bragg grating, the central wavelength of the grating with respect to the laser, the value of the optical distance between the laser and the grating, and the value of the current injected into the laser. In the SDL patent, the central wavelength of the Bragg grating is contained within a 10 nm band around the laser wavelength and the value of the reflection coefficient of the grating 34 is similar to the value of the output face 27 from laser 26. In the preferred embodiment, the width of the band reflected by the grating 34 and its reflection coefficient are such that the return into the laser cavity due to the output face is greater than the return due to the grating 34. Consequently, the grating 34 acts like a disturbance to the emission spectrum of laser diode 26, which has the effect of widening the emission band and thus making the diode less sensitive to disturbances caused by temperature changes or injected currents.
In order to obtain the required effect, in the preferred embodiment the grating 34 has a reflection peak that is located 1 or 2 nm from the wavelength of the diode, a reflection coefficient of 3% which, taking account of coupling between the grating and the diode, produces a return coefficient to the diode equal to 1.08%.
U.S. Pat. No. 5,563,732 issued to AT&T Corp. also describes a pumping laser 13 for an amplifier laser 12 also used to make optical transmissions. This laser 12 is stabilized to prevent fluctuations in the emitted wavelengths caused by parasite reflections from the amplifier laser 12 by means of a fiber grating 14. The inventors have found that the pumping laser 13 is stable if the reflection coefficient from the grating 14 is between 5 and 43 dB.
Experiments carried out by the applicant have shown that the use of lasers stabilized using a fiber grating can have a good influence on the operating stability of the laser and particularly on the stability of the emitted wavelength, but only within certain limits. In particular, the use of lasers stabilized as described in each of the two patents mentioned above cannot produce a laser capable of operating within a temperature range varying from −20° C. to +70° C. as currently required by most users. Therefore, there is a need for such a laser.